test: Added system test for query offset issue

[gkevinzheng]

System test for bug #547 with some additional changes:

1. Added new Mergejoin dataset for bug Bug querying with offset #547 (Googlers see b/352377540)
2. Added options in populate_datastore.py and clear_datastore.py to delete LargeCharacter and Mergejoin kinds from all the namespaces populated by the system tests.

[daniel-sanche]

How much extra runtime does the new test add?

[gkevinzheng]

How much extra runtime does the new test add?

nox -s system -- -k test_mergejoin_query
nox > Running session system-3.8(disable_grpc=False)
nox > Creating virtual environment (virtualenv) using python3.8 in .nox/system-3-8-disable_grpc-false
nox > python -m pip install --pre 'grpcio!=1.52.0rc1'
nox > python -m pip install mock pytest google-cloud-testutils -c /usr/local/google/home/kevzheng/python-datastore/testing/constraints-3.8.txt
nox > python -m pip install six -c /usr/local/google/home/kevzheng/python-datastore/testing/constraints-3.8.txt
nox > python -m pip install -e . -c /usr/local/google/home/kevzheng/python-datastore/testing/constraints-3.8.txt
nox > py.test --quiet --junitxml=system_3.8_sponge_log.xml tests/system -k test_mergejoin_query
..                                                                                           [100%]
-- generated xml file: /usr/local/google/home/kevzheng/python-datastore/system_3.8_sponge_log.xml --
2 passed, 134 deselected in 97.15s (0:01:37)
nox > Session system-3.8(disable_grpc=False) was successful.
nox > Running session system-3.8(disable_grpc=True)
nox > Creating virtual environment (virtualenv) using python3.8 in .nox/system-3-8-disable_grpc-true
nox > python -m pip install --pre 'grpcio!=1.52.0rc1'
nox > python -m pip install mock pytest google-cloud-testutils -c /usr/local/google/home/kevzheng/python-datastore/testing/constraints-3.8.txt
nox > python -m pip install six -c /usr/local/google/home/kevzheng/python-datastore/testing/constraints-3.8.txt
nox > python -m pip install -e . -c /usr/local/google/home/kevzheng/python-datastore/testing/constraints-3.8.txt
nox > py.test --quiet --junitxml=system_3.8_sponge_log.xml tests/system -k test_mergejoin_query
..                                                                                           [100%]
-- generated xml file: /usr/local/google/home/kevzheng/python-datastore/system_3.8_sponge_log.xml --
2 passed, 134 deselected in 103.82s (0:01:43)
nox > Session system-3.8(disable_grpc=True) was successful.
nox > Ran multiple sessions:
nox > * system-3.8(disable_grpc=False): success
nox > * system-3.8(disable_grpc=True): success

I additionally benchmarked how long it takes to populate the dataset, and because of how compact the entries are it takes around 50 seconds to populate the entire dataset.